What happens to the total number of molecules in the system as the pressure is reduced? n2o4(g) 2no2(g) they must stay the same. they will increase. they will decrease. the total will fluctuate (increase and decrease)?

The answer is they will increase

When the pressure is decreased, the equilibrium will shift to the side with the higher amount of molecule. The equilibrium is
N2O4(g) ==> 2NO2(g) 

The side with more molecule count is 2NO2, so the reaction will shift to that direction. The total number of molecules in the system will be increased because of that.

they will decrease.

Explanation:

1) The system given is:

N₂O₄(g) ⇄ 2NO₂(g)

2) This question is answered using Le Chatelier's principle.

When a force or disturbance is introduced into a system in equilibrium, the system moves in the direction that relieves the system of such disturbance.

3) In this case when the pressure is increased the system will try to compensate the increase by reducing the number of molecules, which will result in a relief of the pressure.

4) The system can reduce the number of molecules by reacting in the reverse direction, because 2 molecules of NO₂ combine to produce 1 molecule of N₂O₄. The less the number of molecules the less the pressure.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.